1. Field of the Invention
The present invention relates to an overcurrent protection circuit for, e.g., preventing breakage of a power supply circuit due to an overcurrent or excess current, and also relates to a method of protecting a power supply circuit from an overcurrent flowing through an output transistor of the power supply circuit.
2. Description of the Related Art
The following description sets forth the inventor's knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
Currently, a power supply circuit is used in various electric equipments. In general, a power supply circuit is equipped with various protection functions for preventing breakage of the power supply circuit due to an overcurrent or excess current.
As such a protection function, commonly used is an overcurrent detection function of turning off an output transistor when it is detected that an overcurrent or excess current flows through the output transistor due to, e.g., short circuit.
When an output voltage suddenly reaches a predetermined voltage value at the time of start-up of a power supply circuit, an overcurrent occurs since the output is virtually grounded via a large-capacity capacitor. To avoid this problem, a soft-start function which makes the output voltage gradually raise at the time of start-up of a power supply circuit has become popular.
FIG. 4 shows an example of a power supply circuit equipped with an overcurrent protection function and a soft-start function, which is described by way of example and should not be considered as a prior art. This power supply circuit 100 includes a control circuit 110, a diode 120, a coil 121, a capacitor 122, resistors 123 and 124 which are externally connected to the control circuit 110. The control circuit 110 includes an output transistor 141, an error amplifier 142, an internal electric power source 143, a current source 144, an oscillation circuit 145, a comparator 146, an output control circuit 147, and an overcurrent detection circuit 148.
Now, the general operation of the power supply circuit 100 will be explained. In the output transistor 141, an input voltage is applied to the drain, the control voltage from the output control circuit 147 is applied to the gate, and an output terminal is connected to the source. When the output transistor 141 is turned on depending on the control voltage, an electric current flows through the coil 121 to charge the capacitor 122, which raises the output voltage with respect to a load. On the other hand, when the output transistor 141 is turned off depending on the control voltage, the output voltage with respect to the load drops. The error amplifier 142 is configured to amplify the error between the feedback voltage obtained by dividing the output voltage by resistors 123 and 124 and a reference voltage of the internal electric power source 143 and then output. The comparator 146 is configured to compare the output of the error amplifier 142 with an oscillating signal, such as a triangular wave, outputted from the oscillating circuit 145 and output a PWM signal. The output control circuit 147 is configured to perform PWM control of the output transistor 141 based on the PWM signal. Thus, the power source circuit 100 performs ON-OFF control of the output transistor 141 depending on the error between the feedback voltage and the reference voltage to reduce the error between the feedback voltage and the reference voltage, so that the output voltage reaches the predetermined voltage.
Now, the overcurrent detection function of the power supply circuit 100 will be explained. When the overcurrent detection circuit 148 detects an overcurrent of the output transistor 141 due to, e.g., a short circuit, the circuit 148 outputs a signal for turning off the output transistor 141 to the output control circuit 147 to prevent breakage of the output transistor 141.
The soft-start function of the power supply circuit 110 will be explained below. At the start-up time of the power supply circuit 100, the error between the feedback voltage and the reference voltage is large. Therefore, when PWM control is performed based on the large error, the turn-on time of the output transistor 141 becomes very long. As a result, the possibility of causing an overcurrent is high since the output is virtually grounded to a large-capacity capacitor 122. To avoid this problem, a capacitor 130 is connected to the error amplifier 142. The error amplifier 142 compares one of the reference voltage and the voltage of the capacitor 130 which is lower with the feedback voltage. With this structure, at the time of the start-up of the power supply circuit 100, the potential of the capacitor 130 gradually raises by the current supplied from the current source 144, resulting amplification of the error between the potential of the capacitor 130 and the feedback voltage by the error amplifier 142, which prevents occurrence of an overcurrent (see, e.g., Japanese Unexamined Laid-open Patent Application Publication No. 2004-15881).
In the meantime, the output transistor 141 sometimes flows a high current instantaneously. This is different from an overcurrent due to, e.g., short circuit. However, when the overcurrent detection circuit 148 recognizes as an overcurrent to turn off the output transistor 141, the power supply circuit 100 may causes a problem in operation.
Furthermore, when an overcurrent occurs, the power supply circuit 100 turns off the output transistor 141. However, if the output transistor 141 returns to an on-state immediately after the turn-off state, removal of heat generated in a system in which the power supply circuit 100 is mounted becomes sometimes insufficient.
The description herein of advantages and disadvantages of various features, embodiments, methods, and apparatus disclosed in other publications is in no way intended to limit the present invention. For example, certain features of the preferred embodiments of the invention may be capable of overcoming certain disadvantages and/or providing certain advantages, such as, e.g., disadvantages and/or advantages discussed herein, while retaining some or all of the features, embodiments, methods, and apparatus disclosed therein.